Device for cleaning metal surfaces

ABSTRACT

The device proposed for cleaning metal surfaces consist of pad (12, 34, 91) of insulating material held between a beak-shaped (11, 52, 94) electrode (10, 26, 46) and the metal surface (8) to be cleaned, plus a low-voltage a.c. power supply (2) which is connected via the other electrode to the metal (7). A pump supplies the pad with a highly corrosive, high-density, acid solution. The pad consists of a relatively thick hose or tape. The device has slots (36) into which the gases and vapors produced during cleaning are drawn by an extractor fan (40) and then passed through a washing bottle (41) where they are cleaned. The electrode may have various shapes, and it is possible to replace the tip (28, 94, 98). The electrode is preferably designed with bores through which the acid solution can be fed. The main body (97) of the electrode and the replaceable tip (94, 98) may be coated with a layer (101, 100) of insulating material which prevents short-circuits occurring and concentrates the electrolytic action ant the end surface (102) of the electrode. Woven of felt fabric made of polyetheretherketone is preferably used as the insulating material of the pad. The lifetime and working capacity of the device are maximized by using a double-layer pad (92, 93), the inner layer being made of woven fabric (92) and the other layer of felt (93).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns a device for cleaning metal surfaces after theyhave been worked at high temperatures, that is, an apparatus with whichscabs, oxidation or other marks left by high temperature treatment, suchas the welding of metals and laser or plasma cutting, can be removed.

2. Description of the Prior Art

Prior art comprises a device consisting of a fibreglass pad wound aroundan electrode in the form of a nib. The pad is dipped into an acidsolution of phosphoric acid (81% by weight), quinoline (0.01% by weight)and water and then placed near the surface that is to be treated: apower supply of low voltage alternate current is applied, with theground conductor connected to the piece and the other connected to themetal part of the pad's support. The conditions of an electrolytic cellare thus created in the solution which can then remove the oxide or scabin a short time.

However, with this type of device, the operator is frequently hamperedby technical difficulties with the maintenance and replacement of theinsulating fibreglass material as, being exposed to very hightemperatures, it can burn, and as a considerable quantity of dirt getscaught up in the fabric, hindering the development of suitable electricconditions, also as the pad must frequently be dipped into the acidsolution: if it is not sufficiently damp, the material may becomephysically damaged and the electrolytic action is reduced.

Moreover, the residue remains firmly attached to the treated surfacerequiring several washes before being dried.

Furthermore, care has to be taken with the acid solution, which has ahigh density, to prevent it from dripping onto the operator or onto thefloor when applied to vertical or underside surfaces.

Finally, it is not very practical for the operator to work next to acontainer filled with an acid solution and a great amount of thefibreglass insulating fabric that has to be wound around the nib of theelectrode, in as much as work is not continuous, and thereby notproductive, and it can also be dangerous.

Prior art also comprises a device for cleaning metals with fluid acidsolutions--based on citric acid--which are constantly conveyed to thepad by means of an electric pump placed in the container of the acidsolution.

However, owing to the low power levels employed and the lowaggressiveness of the acid solution, this device has never been widelyused in industry.

Furthermore, the metal electrode through which current flows and whichis wrapped in the pad heats up because it is a good conductor of heat aswell as of electricity: the increase in the electrode's temperature dueto the accumulation of heat in the pad makes the acid solution evaporatequickly and can even cause the pad to burn, causing a short circuitbetween the electrode and the surface being treated. For this reason thepad has to be replaced relatively often.

Finally, none of the prior art devices protect the operator from thefumes generated during the cleaning process.

Such prior art may be subject to considerable improvement with a view toeliminating the said drawbacks, to increasing the safety of the device,its ease of use and to increasing the service life of the pad.

SUMMARY OF THE INVENTION

From the foregoing emerges the need to resolve the technical problem ofinventing a new conformation of the device and a new means for applyingthe electrolytic action to remove the oxides, oxidation and scabs frommetal surfaces, as well as a new conformation of the electrode and thepad with a view to eliminating the said burns.

The invention resolves the said technical problem by adopting a devicefor cleaning metals, comprising a pad of insulating material placedbetween the electrode and the metal surface to be cleaned, a low voltagealternate current power supply applied to the metal with the otherelectrode, an acid solution with which a pad soaked, the pad consistingof a tubular sheath or a strip of insulating material which is more than1 mm thick, characterized in that the material with which the insulatingfabric is made has been improved and consists of polyetheretherketonefabric (with trade name PEEK by Hoechst) and it is wrapped around themetal tip of the nib of electrode.

Also adopting: the said fabric of polyetheretherketone can also be usedin the form of felt.

Also adopting: the said fabric of polyetheretherketone can also be usedin the form of a mesh, coated with a layer of felt of any kind ofinsulating and heat resistant material, this layer being either applieddirectly at the time of production or at a later stage.

Also adopting: the said material of polyetheretherketone coated with alayer of felt, which can also be made of polyetheretherketone.

Also adopting: the said layer of felt being placed on the outer side ofthe layer of polyetheretherketone in relation to the nib.

Alternatively adopting: a device for cleaning metals, comprising a padof insulating material placed between the electrode and the metalsurface to be cleaned, a low voltage alternate current power supplyapplied to the metal with the other electrode, an acid solution withwhich a pad is soaked, consisting of tubular sheath or a strip ofinsulating material which is more than 1 mm thick, characterized in thatthere is a circuit for the supply of the acid solution by means of amanually or electrically driven pump which pressurizes the air in thetank above the free surface level of the acid solution and whereby thesupply is controlled by a valve.

Alternatively adopting: a device for cleaning metals, comprising a padof insulating material placed between the electrode and the metalsurface to be cleaned, a low voltage alternate current power supplyapplied to the metal with the other electrode, an acid solution withwhich a pad soaked, consisting of tubular sheath or a strip ofinsulating material which is more than 1 mm thick, characterized inthat, with a view to improving the safety of the device, there are slitson the electrode near the said pad, connected to a fan to suck up thegases and fumes formed during treatment.

Also adopting: an supply circuit for the acid, with which the very denseand aggressive acid is conveyed, by means of a hand pump or by someother means, from the acid tank which is made of an insulating material,that is, isolating the acid from the environment.

Also adopting: a metal nib, which is hollow or perforated axially,mounted inside the pad at the tip of the electrode to convey the acid.

Also adopting: the said nib wrapped in fabric of insulating materialfolded in two parts and fixed or kept in place with a ring made of anelastic, insulating or thermo-shrink-setting material; this elastic orinsulating ring may be used again after having replaced the said padmade of folded material.

Also adopting: the said nib consisting of a metal body and of areplaceable extremity of insulating material inside which there is atleast one metal body ending near the tip of the nib and which activatesthe electrolytic action.

Also adopting: the said replaceable extremity of the nib of metallicmaterial coated with insulating material on the lateral walls so as toprovide electrical continuity with the body of the nib at its area ofcontact and the non-insulated end surface. The said end surface hasholes through which the acid solution is conducted.

Also adopting: the said insulating coating made up of a 0.1-0.3 mm thicklayer of Teflon® for the prevention of short circuits and a 0.6-1.2 mmthick layer of Teflon for thermal insulation.

Finally, also adopting: the nib made of highly resistant stainlesssteel, that is, both the body and the replaceable metal extremity. Thehighly resistant stainless steel selected for the material of the nibpreferably being AISI 316 stainless steel.

This invention has the following advantages: the operator can apply theacid directly and as often as necessary; even very aggressive solutionscan thus be used, without wasting time dipping the pad and withoutwasting acid solution on the way from the container to the area to betreated. The pad soaks the acid up better and thus heats up less andwears out less through thermo-mechanical use. The nib of the electrodecan have the most varied forms according to the particular workingposition; if the nib is tapered, the sheath can be replaced faster. Theconveyance of the acid solution inside the pad avoids spilling acid orapplying it where it is not needed. As the acid solution is not asconcentrated and more fluid, and as cleaning is simplified due to theease with which residual material is removed, treatment is moreefficient and can be used for production on a large scale and costs arelower than with prior art techniques.

The service life of the sheath, even if made of fibreglass, is greatlyincreased with respect to that found with an entirely metallic electrodedue to the insulating material used for the tip of the electrode or,even if this is made entirely of metal, due to the thick layer ofinsulating material.

Maximum service life can be obtained using a pad with a double layer:the inner one of fabric, such as polyetheretherketone, and the outer oneof felt.

Maximum service life is obtained using polyetheretherketone, because itis extremely resistant to heat and acid. The outer felt layer, made ofany suitable material such as the said fibreglass, keeps the solution onthe area to be cleaned and enables it to adapt to the shape of thepiece, thus greatly increasing the service life of the sheath. The useof sheaths with a double layer of polyetheretherketone, that is, forboth the fabric and the felt, enables best results to be obtained asregards service life, precision and the quantity of work.

Considering the intensive wearing of the active tip of the nib, thereplaceability of the tip of the nib allows the body of the nib to bemade of normal stainless steel, whereas the tip can be made of specialstainless steel which, though more expensive, does not wear out asquickly. As a consequence, the overall cost-effectiveness of thecleaning process is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the invention are illustrated, purely by way ofexample, in the five tables of drawings attached in which FIG. 1 is anoverall view of the device equipped with acid supply circuit and with afabric sheath of insulating material according to the invention; FIG. 2is a magnified view of the nib of the pad wrapped in the sheath; FIG. 3is a hydraulic circuit diagram of the pump for the acid solution; FIG. 4is a hydraulic/pneumatic diagram of the pump pressurizing air in theacid tank, expelling the acid solution; FIG. 5 illustrates the sectionthrough a nib with a truncated-cone shaped metal insert and with sheathmade of the improved insulating material. FIG. 6 is section VI--VIthrough the truncated-cone shaped metal insert; FIG. 7 is a diagramillustrating the intake and solution of the gases and fumes produced byheating the acid; FIG. 8 is a partial view of a device with an improvedreplaceable, quick-release nib, which allows adaptation to the specificworking position; FIG. 9 shows the improved nib wrapped in the sheath ofinsulating material; FIG. 10 is a rear view of the improved nib,illustrating the internal ducts; FIG. 11 is a front view of the saidnib; FIGS. 12 and 13 are views of two nibs especially designed to treatplaces which are difficult to access; FIG. 14 is a view of a nib havinga brush made of insulating material to treat places which are difficultto access; FIG. 15 is a view of the electrode equipped with a tip ininsulating material in accordance with the invention; FIG. 16 is theenlarged section in a longitudinal plane of the nib of the electrode;FIG. 17 is a view of a nib with a brush made of insulating material totreat places which are difficult to access; FIG. 18 is section F--F ofFIG. 17, enlarged and limited to the two small thread-like tubes; FIG.19 is a view of a pad made up of a double layer of special material towrap up the nib; FIG. 20 shows the tip of the nib with its pad wrappedup in a double layer; FIG. 21 is a view of a metal nib with areplaceable tip, which is fixed to the body by means of a screw andwhich is covered by a thick insulating layer.

DETAILED DESCRIPTION OF THE DRAWINGS

The figures show: 1, FIG. 1, the electric conveying apparatus made up ofa transformer 2 to transform power supply current into low voltagecurrent, a selector/breaker switch 3 to adjust the outgoing voltage, twoclamps 4 for the conductors 5 and 6, one connected to the body 7 of thesurface to be treated with pliers 9, and the other connected to the body10 of the electrode which ends in nib 11, in turn wrapped in a sheath12, kept in place by a clip 13; 14, a small tube to convey the acidsolution, inside or outside the insulated handle 15, pumped by means ofa pump 16 from the tank 17 which contains the acid solution 18 and whichis insulated from the rest of the apparatus; 19, the lid of the tankwith an air vent; 20, FIG. 2, the part of the fabric sheath made ofinsulating material covering the nib 11 and the extremity of the body 10of the electrode; with 21, the sheath folded up over the point of thenib and held in place by clip 13; 22, a pump which compresses the air inthe tank 23 and produces pressure that forces the acid solution 18 toflow in the small tube 14; 24, a no-return valve; with 25, a stop valvein the said small tube 14 fitted near the handle of the pad.

The figures also show: 26, FIG. 5, the extremity of the electrode with atruncated-cone shaped section 27, in which a hollow nib 28 is fitted,similarly equipped with a truncated-cone shaped extremity 29 and madeflexible due to the longitudinal cut 30; 31, the internal tube whichcarries the acid solution and which is mounted on the said extremity ofthe electrode with seal 32 and which ends near the axial slit 33; 34,the fabric of insulating material fixed onto the said hollow nib with aring 35 of elastic or thermo-shrink-setting material; 36, FIG. 7, slitsmade near the nib to suck up the gases and fumes produced duringcleaning; 37, the anatomical handle with button 38 to activate the pump16 or the valve 25; 39, a tube to suck up the said gases or fumes,connected to a centrifugal fan 40 which conveys gases into anappropriate moisture filter 41, after which the gases come out of theopening 42 clean; 43, the small tube in which the acid solution isconveyed, and 44, the electric conductor, equipped with a pair of wiresto control the pump 16, if the latter is electric.

The figures also show: 45, FIG. 8, a standardized handle with a bentelectrode body 46, control button 47, similar to button 38; 48, the nibwith a bayonet-joint consisting of the slits 49 of the joint, of a firstsection 50 and of a second, slightly conical section 51 and of atapering nib 52, which ends in open extremity 53 of tube for supplyingthe acid solution; 54 the sheath made of the fabric of insulatingmaterial 55 folded near the nib and fixed onto the tips 56 and 57 bymeans of the clip 58 or elastic element; 59, FIG. 10, the said openingof the tube for supplying the acid solution which is coupled to theelectrode 46 in the ante-hole 60 in such a way that it is sealed and inwhich the solution flows; 61, FIG. 12, a flattening of the nib 52 whichcan hold the part of the folded sheath 54 of triple thickness and whichenable tight recesses to be reached; 62, FIG. 13, a pointed and anglednib to treat internal corners; 63, FIG. 14, a thin, tapering nib with afine brush 64 with stiffened bristles 65 of insulating material: theopening 59 conveys the acid solution to the middle of the brush.

The figures also show: 71, FIG. 15, the body of the tapering metal nibwhich makes up the electrode; with 72 the tip of this nib of insulatingmaterial with an opening 73 to convey the acid solution to the sheaththat makes up the pad, not shown for the sake of clarity; 74, the headsof the screws to fix it to this body, positioned near the end surface S:the stems of these screws extend to the corresponding threaded holes 75,FIG. 16; 76, the opening, through which the acid solution is conveyedinto the said body; 77, the areas of contact between the body 71 and thetip 72: the flexibility of the material employed, that is,fibreglass-reinforced Teflon, is such that no seals need to be used.However, any leakage is collected by the fabric sheath of insulatingmaterial, not illustrated, and which fully envelops the said electrode.

The figures also show: 78, FIG. 17, the body of the electrode with abrush with small tubes of insulating material 79; 80, the openingthrough which the acid solution is conveyed into chamber 81, which is inthe metallic head 82 of the brush in which the said small tubes arefitted; 83, FIG. 18, the block that hold said small tubes of insulatingmaterial. The figure only shows two small tubes, but they suitably coverthe whole surface of the said head; 84, the openings of these smalltubes, through which the acid solution is conveyed from the said chamber81 into the metal recess to be cleaned.

Finally, the figures also show: 91, FIG. 19, the pad, made up of a layerof fabric of insulating material 92, possibly of polyetheretherketone,and a layer of insulating material of felt 93; P, the width of the stripof the pad, which is wider than the nib for which it intended; 94, FIG.20, the tip of the nib seen from the side, around which the two sides ofthe pad illustrated in the preceding figure are wrapped, with the layerof material 92 on the inside and the layer of felt 93 on the outside;97, a metal nib with a replaceable metal tip 98, which is fixed ontothis nib by means of screws 99; 100, the insulating layer of thereplaceable tip, which can be of a considerable thickness; 101, the thinlayer, for protection against short circuits, on the body of the saidnib 97; 102, the metal surface where wearing of the metal by theelectrolytic cleaning is concentrated; 103, the opening to convey theacid solution.

The device for electrolytic cleaning of metal surfaces uses an acidsolution with relatively small percentages of phosphoric acid (52.5% byweight) and quinoline (0.01% by weight) in water: the completely fluidacid solution, unlike the high density solutions used so far, enablesthe use smaller quantities of solution conveyed directly into the pad ofthe sheath 12 or 54 or of the pad 34 of the hollow nib 28, thuspreventing the solution from dripping onto the surrounding areas andleaving relatively few and easily washable surfaces after cleaning,thereby enabling considerable savings to be made.

The solution is conveyed continuously so that the pad 12, 34 or 54 isalways soaked.

The electrolytic action of the current flowing through the acid solutionis remains constant, so the operator does not have to interrupt work todampen the pad, but simply presses the button 25, 38 or 47 to convey thesolution to the handle 37, 45.

The hydraulic circuit shown in FIG. 3 operates with a pump 16, which canbe manual, operated directly by the operator by means of a small rubberpressure vessel, or driven by means of a low voltage electric motor: inthe case of a system that works with air pressure, the tanks 17 or 27are made of insulating material, in that the column of solution, as itis conveyed to the electrode, constitutes a live conductor from theelectrode to the inside of the tank. The pneumatic-hydraulic circuit ofFIG. 4 also operates by means of a pump 22, which can be manual,generating pressure in the tank 23; as the solution travels to the nibthrough valve 25, so the pressure in the tank decreases, making itnecessary to regenerate the pressure by means of the said pump 22. Thepressure can be maintained automatically with a pressure switch--notshown --that measures the pressure inside the said tank.

The tubular sheath 12, 54 made of a fabric of insulating material ismounted by introducing the first tip 20, 56 into the nib 11, 52 until itfully covers it. The sheath is twice as long, so that it can be foldedwith the other end 21, 57 in the same position as the first tip. Theclip 13, 58 is placed in such a way as to fix both tips at the same timeonto the body of the nib 11, 48.

The hollow nib 28, which is equipped with a fixed layer of fabric ofinsulating material 34 mounted beforehand, is fitted or replaced in asimilar but easier way. This nib is pressed onto the conical element 27of the tip 26 of the electrode, with its truncated-cone-shaped extremity29. It is replaced in a similar way by completely removing the said niband fitting another nib. The hollow nib with a truncated-cone shapedratchet can take on various forms, as can be seen from the nibs in FIGS.12 and 13 or with the brush in FIG. 14.

The snap-fit nibs have a seal between the ante-opening 60 and the bentelectrode 46 to prevent leakage of the acid solution and to convey it tothe opening 59 with no waste. With the flattened shape 61 illustrated inFIG. 12, the nib can have a tubular-shaped sheath folded in the saidflattening in such a way as to be much thinner than with the normal nib48. Finally, the pointed and angled shape 62 of the nib makes itpossible to reach recesses and corners in tight places which aredifficult to access.

Furthermore, the nibs can have a screw-fit joint instead of a snap-fitjoint.

Nibs 63 fitted with a brush 64 with bristles of insulating material 65are used by running them longitudinally or transversally over thebutt-weld, the said bristles, stiffened with a coating of Teflon orsimilar material, being able to reach the bottom of the notch betweenthe wall and the butt-joint and can clean it thoroughly.

The pistol with the slits 36 to suck up the gases and fumes enables theoperator to work without danger. The air that is sucked up, whichcontains the gases and fumes, is conveyed through the moisture filter41, which is adapted according to the products used to clean metalsurfaces with acid solutions.

While cleaning, the tip 72 does not conduct the electric current: thisoccurs instead through the heads of the screws 74 which are fitted sothat they are parallel with the surface at the end of the tip S. Thecurrent is conducted from these screw heads to the sheath soaked in theacid solution, thus activating electrolytic cleaning. As the tip 72 doesnot conduct electricity, it does not heat up during the electrolyticaction of the sheath on the surface to be treated, and, consequently,the sheath is not irreparably damaged by burning.

The electrode with the body 78 fitted with a brush made up of smalltubes 79 of insulating material operates by conducting the electriccurrent into the acid solution from the said metal body 78 to thesurface of the notch to be treated, getting at the inner-most corners ofthis recess.

Tests carried out with the tip of the electrode made offibreglass-reinforced Teflon have shown that the sheath has a servicelife which is longer than the common sheaths with tips made entirely ofmetal, by approximately a work shift.

The pad 91, improved with a double-layered material 92, 93, the outerlayer of which is made of felt 93, is used by wrapping up the tip of thenib and holding the pad with a fastening ring 96. The pad does notnecessarily overlap the side of the tip perfectly, whilst the middlepart of the said pad covers the active area 102 of the metal tip 98, theinsulating coating 100, preferably consisting of a 0.6-1.2 mm thicklayer of Teflon®, avoids electrolytic action at the sides of the tip,whereas the totally uncovered face is the means for the said action.Less advantageously, the tip may be coated only by a thin layer ofinsulating material, for example a 0.1-0.3 mm-thick layer of Teflon,which also protects it from damage and short circuits on the sides, asis the case for the body of the nib 97 with the coating 101.

Even though it is possible to use various insulating materials for thetwo layers 92, 93 of the said pad, the double-layered pad 91 lastslongest if the inner layer 92 of the material is made ofpolyetheretherketone and the outer layer 93 is made ofpolyetheretherketone felt. The double-layered strip can be produced byjoining the two layers afterwards or by placing one layer over the otherdirectly at the time of production.

The worn pad can easily be replaced by removing the fastening ring 96and changing the strip at the tip of the pad. The ring 96 of insulatingmaterial can be re-used a number of times.

After the metal tip 98, equipped with a thick insulating layer 100, oreven not so thick, is worn out and can no longer fulfill its cleaningfunction as well as required, it is replaced by removing the screws 99.

The longest service life for the metal tip of the nib is achieved usingstainless steel of AISI 316 quality; with all other types of stainlesssteel, costs are reduced, but wear is increased.

With prior art devices the technical problem of wear did not arise dueto the low power levels employed and consequently the low productivityachieved with these devices.

In practice the materials, dimensions and details of execution maydiffer from, but be technically equivalent to, those indicated withoutdeparting from the juridical domain of the present invention.

We claim:
 1. Device for electrolytic cleaning metal surfaces of aworkpiece using an acid solution after they have been worked at hightemperatures, said device comprising: a fabric pad of insulatingmaterial adapted for placement between a metal tip of a nib of a firstelectrode (10) and a metal surface (8) to be cleaned, a low voltagealternate current power supply (2) adapted for application to the metalsurfaces (7) through a second electrode (9); said pad being of a tubularsheath (12) or a strip of an insulating polyetheretherketone fabric ofmore than 1 mm thick covering said metal tip of said nib of said firstelectrode; wherein said pad is arranged to be supplied with the acidsolution.
 2. A device for cleaning metals, as claimed in the previousclaim 1, wherein said polyetheretherketone fabric is in the form offelt.
 3. A device for cleaning metals, as claimed in claim 1, whereinsaid polyetheretherketone fabric is coated with a felt layer ofheat-resistant insulating material.
 4. A device for cleaning metals, asclaimed in claim 3, wherein said polyetheretherketone fabric is in theform of a mesh coated with a felt layer made of polyetheretherketone. 5.Device for cleaning metals, as claimed in claim 3, characterized in thatthe said layer is in the form of felt (93) placed on the outside of thelayer of material (92) of polyetheretherketone in relation to the nib(94).
 6. A device for cleaning metals, as claimed in claim 1, whereinsaid nib is mounted on the tip of the electrode and is either hollow oraxially perforated, and through which the acid solution is may beconveyed.
 7. Device for cleaning metals, as claimed in previous claim 1,characterized in that the said nib is wrapped in fabric made ofinsulating material which is folded in such a way as to from two parts(34, 91) and is fixed or held in place by a ring (35, 96) of elastic,insulating or thermo-shrink-setting material; it being possible tore-use the said elastic or insulating ring many times after replacingthe pad of folded fabric.
 8. Device for cleaning metals, as claimed inclaim 1, characterized in that the said nib consists of a metal body(71) and a replaceable tip of insulating material (72), which has insideit at least one metallic element (74) with an extremity near the tip (S)of the nib and which may activate electrolytic action.
 9. Device forcleaning metals, as claimed in claim 8, characterized in that the saidreplaceable tip (94, 98) of the nib (97) is made of a metallic materialcoated with an insulating material (100) on the lateral walls so thatthere is electric continuity in the area of contact with the body of thenib and on the uninsulated end surface (102); the said end surface hasholes (103) through which the acid solution may be is conveyed. 10.Device for cleaning metals, as claimed in claim 9, characterized in thatthe said insulating layer (101) consists of a 0.1-0.3 mm-thick layer ofpolytetrafluoroethylene for protection against short circuits. 11.Device for cleaning metals, as claimed in claim 10, characterized inthat the said insulating layer (100) consists of a 0.6-1.2 mm-thicklayer of polytetrafluoroethylene for thermal insulation.
 12. Device forcleaning metals, as claimed in previous claim 8, characterized in thatthe material of which the nib is made, is a highly resistant stainlesssteel.
 13. Device for cleaning metals, as claimed in claim 12,characterized in that the replaceable tip is made of high strengthstainless steel AISI
 316. 14. A device for electrolytic cleaning metalsurfaces of a workpiece using an acid solution after they have beenworked at high temperatures, said device comprising: a fabric pad ofinsulating material adapted for placement between a first hand-holdableelectrode (10) and a metal surface (8) to be cleaned, a low voltagealternate current power supply (2) adapted for application to the metalsurface (7) through a second electrode (9); said pad being a tubularsheath (12) or a strip (34, 91) of a fabric of insulating material morethan 1 mm thick; a circuit with a pump connected to a tube (14) and astop-valve (25) for supplying said pad with the acid solution from aseparate tank (23).
 15. A device for electrolytic cleaning metalsurfaces of a workpiece using an acid solution after the surfaces havebeen worked at high temperatures, said device comprising: a fabric padof insulating material adapted for placement between a tip of a firstelectrode and a metal surface to be cleaned, a low voltage alternatecurrent power supply adapted for application to the metal surfacethrough a second electrode; said pad being a tubular sheath or a stripof a fabric of insulating material more than 1 mm thick; and slits onsaid first electrode near said pad connected to a fan adapted toevacuate gases and fumes formed during the cleaning of metal surfaces;wherein the evacuation of the gases and fumes improves the safety in theusage of the device.
 16. A device for cleaning metals, as claimed inclaim 7, further comprising a supply circuit to convey the acidsolution, by means of a pump, from a tank that is adopted to contain thesolution.